It is known in the prior art to separate oxygen from air using a molten stream of an oxygen acceptor comprising a liquid containing alkali metal nitrite and nitrate salts. This fundamental chemical air separation is set forth in U.S. Pat. No. 4,132,766. At least some energy of compression of the air feed to such a separatory process is recovered by the expansion of oxygen depleted gas from the separatory process.
The coproduction of oxygen and nitrogen as relatively pure products of a chemical separation utilizing the alkali metal salts of nitrite and nitrate materials is also known. In U.S. Pat. No. 4,287,170, air is contacted sequentially with such alkali metal salts, and then residual oxygen is scavenged from the oxygen depleted effluent with an absorption media, such as manganese oxide. At least some energy of compression for the feed air is recovered by expanding the nitrogen product to a lower pressure.
This prior art (U.S. Pat. Nos. 4,132,766 and 4,287,170) is not uniquely integrated by heat exchange with a combustion process to co-produce a high temperature process stream, oxygen and nitrogen.
U.S. Pat. No. 4,340,578 discloses a method for producing oxygen with a chemical absorbent solution of molten alkali metal nitrite and nitrate salts wherein the salt solution contains additional oxides in low concentration, and the oxygen depleted effluent from the chemical separation is combusted with fuel and expanded to recover power in two stages. The combustion effluent is heat exchanged with the air feed and the oxygen product to elevate the air feed to absorption conditions. The molten salt absorbent solution is depressurized to release the reversibly contained oxygen therefrom and provide an oxygen product.
It is also known to separate oxygen from air using cryogenically low temperatures and subsequently to combust the waste nitrogen along with a portion of the air feed to the cryogenic separation with expansion of the combustion effluent to recover power for the process. Such a process is set forth in U.S. Pat. No. 4,224,045.
The net generation of power and the production of oxygen has also been disclosed in U.S. Pat. No. 4,382,366 wherein air is compressed and separated in a cryogenic low temperature distillation column. The waste nitrogen is combusted with fuel and expanded through a turbine to recover power for the compression of feed air to the cryogenic separation, as well as to pressurize oxygen product and to generate power. The combustion effluent may also be utilized to raise steam to enhance power generation.
The prior art (U.S. Pat. Nos. 4,340,578; 4,224,045; 4,382,366) provides for energy recovery from an air separation process by combusting the waste nitrogen directly with fuel followed by expansion of the combustion effluent to recover power.
The net generation of power by the recovery of heat from a low pressure combustion process by heat exchange to produce a high temperature pressurized process stream, and subsequently recovering power from the process stream by expansion in a power recovery turbine is exemplified by the well known Rankine cycle. Steam is the preferred working fluid. For efficient high temperature recovery of heat, higher steam pressure, about 400 psia to 2500 psia, is needed in order to provide the maximum power recovery in the isentropic expansion stage of the Rankine cycle. The metallurgical strength limit of the heat exchangers metal tubes becomes a problem since a high wall temperature tube cannot withstand the high pressures. Efficient steam generation heat recovery from atmospheric pressure combustion gases uses pressure differentials across the heat exchanger metal tubes in the range of 20 to 170 atmospheres.
U.S. Pat. No. 3,310,381 discloses the recovery of oxygen from air using a suspension of solid absorbent in a liquid carrier in a cocurrent contact of air and absorbent. Temperatures above 932.degree. F. are recited for the system which uses barium oxide and barium peroxide.
The patent process is a continuous version of the Brin process using a pressure and temperature swing cycle. Feed air cocurrently contacts the barium oxide/barium peroxide suspension in an absorber which heat exchanges with an external heat exchange fluid. The absorber operates at approximately 1112.degree. F. and a pressure slightly above atmospheric pressure. The oxidized acceptor is further heated to approximately 1472.degree. F. in a heater. The high temperature oxidized acceptor is reduced in pressure and desorbs oxygen with attendant reduction in temperature to 1328.degree. F. The partial pressure of the oxygen in the acceptor is determined by temperature because the barium oxide and barium peroxide are always present in the suspension of acceptor.